1. Technical Field
The present disclosure relates to a microelectromechanical device having an oscillating mass and a method for controlling a microelectromechanical device having an oscillating mass.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has increasingly spread in various technological sectors and has yielded encouraging results especially in providing inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS systems of this type are usually based upon microelectromechanical structures comprising at least one mass connected to a fixed body (stator) by means of springs and movable with respect to the stator according to pre-set degrees of freedom. The movable mass and the stator are capacitively coupled through a plurality of respective comb-fingered and mutually facing electrodes so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors; whence it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the force applied. On the other hand, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. In addition, for providing electromechanical oscillators the frequency response of MEMS inertial structures is exploited, which is typically of a second-order low-pass type with one resonant frequency.
MEMS gyroscopes have a more complex electromechanical structure, which comprises two masses that are movable with respect to the stator and are coupled to one another so as to have a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is kept in oscillation at the resonant frequency. The other mass is drawn in the (translational or rotational) oscillatory motion and, in the event of rotation of the microstructure with respect to a pre-determined gyroscopic axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the driven mass, which is capacitively coupled to the fixed body through electrodes, as likewise the driving mass, operates as an accelerometer, which enables detection of the Coriolis force and acceleration and hence makes it possible to trace back to the angular velocity.
In gyroscopes, as likewise in other devices, the transduction of the quantities requires that the movable mass or the system of movable masses be maintained in oscillation at a given frequency. Clearly, upon turning-on of the device (power-on) or at exit from low-consumption configurations (power-down) a start-up transient, during which the movable mass is brought up to the given frequency, occurs before the movable mass or the system of movable masses reaches a stable condition of oscillation.
In the start-up transient, the oscillatory motion is forced through start-up components, which supply a fixed amount of energy, normally in the form of a pulse train of pre-set duration, sufficient to reach the nominal operating frequency. Once the transient is exhausted, the start-up components are de-activated, and the oscillation is maintained by the devices that maintain normal operation.